1. Field of the Invention
The present invention relates to an optical information recording disk.
2. Description of the Prior Art
Conventionally, it is known that a DRAW (Direct Read After Write) type of an optical recording disk is constructed in the form of the so-called air sandwich structure as shown in FIG. 1. This optical recording disk usually comprises a transparent disk 1 and a protection disk 2 each being made of plastic materials such as acrylic resin, polycarbonate resin, etc. A major surface of the transparent disk 1 is provided with a transferring layer 3 made of an ultraviolet ray setting resin for carrying pre-addresses and pre-grooves. Each of the pre-addresses is a series of very minute pits representing an address assigned to a pre-groove following thereto. The pre-groove is a shallow groove extending circumferentially on the transparent disk 1. A recording layer 4 made of a material having a property of light energy absorption such as an organic cyanine dye, is formed on the major surface of the transferring layer 3. A light spot of a laser beam may be aligned onto a selected pre-groove so as to write and/or read any information on the recording layer 4 at the selected pre-groove while referring to the pre-address corresponding to the selected pre-groove. The transparent disk 1 and the protection disk 2 are adhered by an adhesive agent layer 7 via inner and outer spacers 5 and 6 so as to face each other while the recording layer 4 being oriented inwardly.
In the prior optical recording disk, it was desired to make thinner the respective elements of the optical recording disk in order to cause a recording light beam to pass efficiently through the transparent disk 1, and further to facilitate the handling of the optical recording disk. However, it is difficult to obtain the rigidity of such a thin disk enough for avoiding any deformation of the thin disk during its manufacturing process and/or usage.
Therefore, another prior optical recording disk having a construction as shown in FIG. 2 has been developed. This optical recording disk comprises the transparent disk 1 and the protection disk 2 between which a reinforcement disk 8 is disposed so as to accomplish a rigidity sufficient for preventing from the deformation thereof. In the optical recording disk as shown in FIG. 2, the inner and outer spacers 5 and 6 are integrated with each other by the reinforcement disk 8.
However, even with the above-mentioned arrangement, the warp of this disk still occurs due to changes of temperatures and humidities under various using conditions, because the rate of swelling of the transferring layer 3 made of the cured ultraviolet ray setting resin is lower than that of the transparent disk 1 made of acrylic resin. Since furthermore the coefficients of thermal expansions of the transparent disk 1, the protection disk 2 and reinforcement disk 8 are different from each other, it has been a problem that the warp of the optical recording disk occurs due to changes of temperatures.